Various types of starter/generator systems operable in both starting and generating modes have been utilized. One such system is a variable-speed, constant-frequency (VSCF) system in which a brushless, three-phase synchronous generator operates in a generating mode to convert variable-speed motive power supplied by a prime mover into variable-frequency AC power. The variable-frequency power is rectified and provided over a DC link to a controllable static inverter. The inverter is operated to produce constant-frequency AC power, which is then supplied over a load bus to one or more loads. The generator can also be operated as a motor in a starting mode to convert electrical power supplied by a power source into motive power which may in turn be provided to the prime mover to bring it up to self-sustaining speed.
One example of a starter/generator system operable in both starting and generating modes is described in U.S. Pat. No. 3,902,073 to Lafuze. In that patent, power is provided to the synchronous generator during the start mode via a source of 400 hertz AC power.
In certain circumstances, such as during an in-air aircraft engine restart, it is advantageous to provide power to the starter/generator system via a source of battery power provided on the aircraft having a relatively low voltage, such as 24 or 28 volts. In such cases, it is desirable to maximize the efficiency of the system during the start mode to minimize the size and weight of the battery required.
In a starter/generator system powered by a battery during the start mode, the battery voltage is typically boosted by a DC/DC converter to a constant higher voltage, such as 270 volts, and is then provided via a DC link to a pulse-width modulated (PWM) inverter coupled to deliver power to the main generator portion armature winding. As the prime mover is accelerated, the frequency and duty cycle of the PWM inverter are gradually increased. Although such a starter/generator system may be generally satisfactory, the efficiency of the system is not optimal due to the DC/DC conversion of the relatively low battery voltage to a much higher constant DC link voltage and the variable duty cycle of the PWM inverter.